This invention relates to a programmable controller of a scanning type having execution steps represented by a relay ladder diagram type programming.
Most of recent programmable controllers are designed so that sequence circuit diagrams represented by relay ladder diagrams and user programs are repeatedly executed at high speeds. Such a programmable controller is called a scanning type.
When the programmable controller of this kind is actually used, there are provided, in many cases, the following functions in order to monitor the control status by configuration of the user.
There is provided a function in which for example, when the progress in sequence control reaches a certain specific condition, it is indicated by some kind of display. This function is effective to grasp the processing situation of the sequence control.
There is provided a further function in which for example, if an abnormal condition which would not occur in a normal sequence control should occur, the abnormal condition would be indicated. Also, there is provided a function which gives an alarm or stops the operation of a certain device when such an abnormal condition as described above occurs. These functions are effective to promptly indicate the occurrence of abnormal conditions so that appropriate measures can be taken.
In the conventional programmable controller, the above-mentioned status monitoring functions have been realized by making programs in a manner similar to the sequence control program. That is, in order to realize the display of the progressing condition of sequence control and the processing of the abnormal conditions corresponding thereto, the user has incorported predetermined display circuits and abnormal condition processing circuits into user programs, for example, in accordance with the programming system by the relay ladder diagram.
However, this system poses a problem in that if a scale of the programs for the display circuits and abnormal condition processing circuits occupied in the user programs increases, an area of the user program that may be actually used effectively decreases.
Furthermore, if the number of input/output contacts in connection with the display circuits or the like increases, it becomes extremely cumbersome to accurately recognize the logical relation therebetween to constitute the desired display circuits or the like. It is not possible for those other than persons familiar with the programmable controller and control object system to do so.
There is a further problem in that if a number of output relays have to be used for the above-described display circuits and abnormal condition processing circuits, the number of output relays available for the original purpose of control decreases.